Dougie MacLean's love song to his homeland has become an anthem of Scottish pride but it was written in just 10 minutes on a beach in France when he was in his early 20s.
Caledonia is belted out by football crowds, pub regulars and even wedding guests and feels like a traditional tune that has been around for generations.
But this year marks four decades since the song featured on MacLean's debut album.
The Perthshire songwriter, who plays Celtic Connections on Wednesday night, told BBC Scotland: "I wrote it in my very early 20s on a beach and then finished it off in a youth hostel somewhere around Paimpol in Brittany.
"I was genuinely homesick and it was just a little song I wrote."
He says he had been busking around Europe with three Irish guys but the day after writing the song - with its lyric "Caledonia you're calling me and now I'm going home" - he set off back to Scotland.
In the years since, it has become part of Scotland's "common culture", says MacLean, often being talked about as a candidate for the national anthem.
"I was very young when I wrote it so the words are very simple," he says.
"It is a very honest, authentic, genuine little love song for the place where I grew up, maybe that's why it pushes buttons in people."
The song's effect on expatriate Scots was summed up by Desert Island Discs presenter Kirsty Young, who chose it to mark the programme's 75th anniversary last year.
She said: "This song for me exquisitely captures the sensation of having Scotland in your soul and only ever feeling truly complete when you are there."
MacLean, who is now 63, grew up in the village of Butterstone near Dunkeld in Perthshire and started playing fiddle with traditional Scottish music band the Tannahill Weavers before going solo.
He was still finding his way as a songwriter when he wrote Caledonia and says that if he tried to write it now it would be much more complex lyrically and musically.
"I probably would not be able to write something like Caledonia now, which is fascinating as a songwriter," he says.
Caledonia - Music and Lyrics by Dougie MacLean
I don't know if you can see the changes that have come over me
In these last few days I've been afraid that I might drift away
So I've been telling old stories, singing songs, that make me think about where I come from
That's the reason why I seem so far away today
Let me tell you that I love you and I think about you all the time
Caledonia you're calling me and now I'm going home
But if I should become a stranger you know that it would make me more than sad
Caledonia's been everything I've ever had
I have moved and I've kept on moving, proved the points that I needed proving
Lost the friends that I needed losing, found others on the way
I have tried and I've kept on trying, stolen dreams, yes there's no denying*
I have travelled hard sometimes with conscience flying somewhere in the wind
Now I'm sitting here before the fire, the empty room the forest choir
The flames that couldn't get any higher they've withered now they've gone
But I'm steady thinking, my way is clear and I know what I will do tomorrow
When the hands have shaken and the kisses flow then I will disappear
'Rude not to'
Forty years after recording the song, MacLean says he has to sing it wherever in the world he goes, from the Yukon delta in Alaska to the Northern Territories of Australia.
He says: "It would be very rude of me to do shows without singing it because it means so much to so many people.
"I still enjoy singing it. I can still transport myself back to being young 20-year-old that wrote it."
Over the years the song has been covered by musicians from Eddi Reader and Paolo Nutini to Amy Macdonald and Ronan Keating.
It was used for a television advert for Homecoming Scotland 2009 but the most famous version was sung by Frankie Miller for a Tennent's Lager commercial in the early 90s.
MacLean says: "I remembered them asking me about the ad and I went down thinking 'I'm not letting anyone use my song to sell beer'.
"Then I saw the ad and said to myself 'this is about Scottish self-confidence, go ahead, use the song'."
MacLean says there have been some "horrendous" cover versions over the years but generally he loves to hear other people singing it.
"I walk down the street and there are buskers singing it," he says.
"I have friends who play in pubs who say when they are dying a death they just whip that song out and the whole pub is back on their side again.
"That's lovely. As a songwriter you only need one little thing like that to vindicate everything that you have done."
The 40 years since he wrote Caledonia have seen MacLean become one of Scotland's most successful folk musicians.
In the past decade he has been inducted into the Scottish Traditional Music Hall of Fame, received an OBE and a Lifetime Achievement accolade at the 2013 BBC Radio 2 Folk Awards.
For more than a decade he and his wife ran their own festival in Perthshire and he now broadcasts to the world every month from the studio he created at the village school in Butterstone.
The Caledonia of his most famous song's title refers to the name the Romans gave to the country beyond the northern boundaries of Britannia that they failed to control.
It is a romantic name for Scotland with connotations of history, mystery and resistance.
MacLean voiced his support for independence during the Scottish referendum in 2014 and sang Caledonia at Yes rallies but he says it is up to every individual to decide what the song means.
"It transcends a lot of that stuff," he says.
"As a songwriter you try to have universal themes in the songs so other people can sing it using their own background and their own connection to that universal truth.
"That's when you feel like you have got a good song."